In order to form, through full-enclosed forging, a product including a boss portion radially provided with shaft portions, such as a trunnion (tripod member) for constant velocity universal joints and a cross spider for universal joints, a full-enclosed forging die is used.
As illustrated in FIG. 9, the full-enclosed forging die includes openable/closable dies 1 and 2 and punches 4 and 5 arranged so as to be operable on a central axis of the dies 1 and 2. Specifically, through pressing with the punches 4 and 5 under a state in which the dies 1 and 2 are closed, a cavity 9 is formed, which corresponds to shapes of shaft portions 7 and a boss portion 8 of a product 6. Thus, as illustrated in FIG. 10A, a billet (material) 10 (refer to FIG. 10A) is put into the cavity 9, and then clamping is performed. After that, the billet 10 is pressed with the punches 4 and 5 so as to be plastically deformed. As a result, as illustrated in FIG. 10B, the product 6 can be formed, which is provided with the boss portion 8 and the shaft portions 7.
Specifically, through forging of the billet 10 put into the dies 1 and 2, the billet 10 having a columnar shape and a curvature radius R2 as illustrated in FIG. 10A, the product 6 can be formed, which includes the shaft portions 7 each having a leading end surface 7a with a curvature radius R2′ larger than R2.
By the way, in the full-enclosed forging die, processing load sharply increases in a sealed state, which may lead to a risk of breakage of the die and shortening of a life thereof. As a countermeasure, conventionally, there has been proposed to set a shaft-molding portion to be longer than a required shaft-portion length so that a relief portion is provided at a shaft-leading-end portion (Patent Literature 1).
However, in the conventional die including the shaft-molding portion in which the relief portion is formed, a material obtained through extrusion of the billet pressed with the punches into the shaft portion is liable to be fluidized on a central portion at the leading end surface of the shaft portion and is less liable to be fluidized on a peripheral portion thereat. As a result, a shaft portion is formed, which has a leading end surface with a curvature radius smaller than a curvature radius of a leading end surface of a regular shaft portion. In this manner, in the conventional die provided with the relief portion formed at a shaft molding portion, there occurs “shear drop” which represents retraction of an outer peripheral side toward a proximal end side in a shaft-portion axial direction relative to the leading end surface of the regular shaft portion.
Thus, in order to secure a length of the shaft portion while forming the shaft portion with high accuracy with the die, an extra material is required by an amount of the “shear drop”. By the way, products molded by forging with the full-enclosed forging die constitute inner joint members for constant velocity universal joints and universal joints. Thus, in order to compactify and weight-reduce the constant velocity universal joints and the universal joints using the products, it is necessary to remove a leading end of the shaft portion by a machining process.
Further, in order to prolong lives during use of the constant velocity universal joints and the universal joints in which the products are incorporated, and to suppress vibration and noise, it is required to enhance strength and hardness of the products by heat treatment, and to form shaft-portion outer peripheral surfaces of the products with accuracy higher than that in a forging process. Thus, it is necessary to finish the products with high accuracy by a machining process after the heat treatment. In anticipation of the machining process after the heat treatment, in some cases, the shaft leading end is removed by a machining process prior to the heat treatment so that an intersection surface of a removal surface and the shaft-portion outer peripheral surface is used as a reference plane for phase determination at the time of a high-accuracy machining process on the shaft-portion outer peripheral surface. Therefore, it is necessary to form the intersection surface with high accuracy.
Under the circumstance, conventionally, there has been proposed to perform, prior to a step of molding a product shape (referred to as mid-molding), a preliminary molding step of increasing a curvature radius of a part corresponding to the shaft portion at the time of mid-molding. With this, at the time of molding of the shaft portion in mid-molding, shear drop can be reduced even under a state in which the central portion of the shaft portion is liable to be fluidized in comparison with the peripheral portion thereof and in which the material does not comes into contact with the relief portion. As a result, the constant velocity universal joints and the universal joints can be compactified and weight-reduced (Patent Literature 2).